Page:Popular Science Monthly Volume 4.djvu/541

between the crust and solid nucleus has solidified, will eruptions of lava cease to flow from volcanoes.

As time progressed this congealed crust would increase in thickness. Being unyielding, there would be stamped upon it, as plainly as the form of a pitcher by the moulder, the peculiar flattening of the earth, as determined by the rate of rotation. As soon as the internal fires were concealed, the rotation of the earth would give rise to the alternation of day and night—not, certainly, of the same length as now, since the bulk of the sphere was greater, and with a reduction of size the tendency is to an increase in the rate of rotation. But, with the thick atmosphere, the days must have been dark and gloomy.

At the present day the attraction of the sun and moon produces the phenomena of the tides. As the crust is rigid, only the water upon it can now be moulded into different shapes. But, when the whole earth was pliable, its form must have varied daily, much more symmetrically than at present. As the outer envelope stiffened by cooling, tidal waves would form with great difficulty, and eventually the crust would become too rigid to be affected. Perfect rigidity was not attained during the whole inorganic period. While thin, the crust may have been broken by the attraction, and the liquid oozed out through the crevices, overflowing the surface, and returning at low tide. So great is the power of tidal attraction that a rigid envelope, hundreds of miles in thickness, would be fractured by it. The rents formed were like the faults observed in the strata of the organic periods. More or less fracture probably attended every tidal attraction, until the ocean covered the surface, and presented a material easily modulated.

Age of Chemical Changes.—Following the age of igneous fluidity there succeeded another of great interest. It opens with the surface dry, rough, and slaggy; the interior in intense fusion, and the atmosphere containing all the water of the ocean with numerous volatile compounds. Before its close an ocean is formed, most of the gases have left the atmosphere, and chemical agencies acted with great intensity, and so universally as to characterize the period. The falling of the primeval rain dissolved acids in the air, and poured upon the elements never exposed to moisture streams of acidulous waters, well fitted to dissolve out large portions of the original crust.

In order to ascertain the character of this primitive rock, we must adopt the method suggested by Sterry Hunt, in his lecture before the Royal Institution of Great Britain, and consider what changes would result if intense beat should now act upon the crust. The water everywhere would be evaporated, leaving behind its saline impurities. All the carbon in living plants, and the immense supplies of coal stored up in the earth, would become converted into carbonic acid; the siliceous parts, fused with limestones and other rocks, would make silicates of lime, magnesia, etc., and expel the carbonic acid. The sulphur would form sulphurous acid with oxygen, changing eventually